1. Field of the Invention
The invention relates to an apparatus for producing a mineral wool nonwoven fabric including a shroud comprising fiberizing means and a conveyor means for transporting the mineral wool nonwoven fabric, as well as to a method for producing mineral wool nonwoven fabric. The invention relates furthermore to a mineral fiber product having a defined density distribution across the thickness.
2. Prior Art
The intention in producing mineral wool nonwoven fabric is obtain a product of best possible quality for the least amount of energy required. In mineral wool nonwoven fabric production the raw materials are fed molten to a fiberizing means which generates the mineral wool fibers. The mineral wool fibers are discharged into a shroud and deposited on the conveyor means. The bottom conveyor means is usually an air-permeable circulating transport belt. Located under the transport belt is a suction device for generating a specific vacuum.
Since the fiberizing means typically employed in this field convey the vitreous fibers emerging centrifugally from a body in high-speed rotation by a strong downflow of air, a considerable proportion of the air flow is blown into the shroud. This air flow impinges the conveyor means arranged at the bottom of the shroud and is deflected upwards thereby in a zone of high turbulence, resulting in return flow within the shroud. It is this return flow that tends to return upwards the mineral wool fibers already deposited on the conveyor means. To counteract this effect a high-power suction blower needs to be provided so that the mineral wool fibers deposited on the conveyor means are held in place by an adequate vacuum. This vacuum needs to be sufficient so that also in the case of thick mineral wool layers on the conveyor means the topmost layers still remain in place.
When it is desired to produce a relatively thick mineral wool nonwoven fabric several fiberizing means are arranged in a shroud in the conveying direction of the conveyor means. This, however, increases the energy consumption of the suction device since the thicker the layers of the mineral wool nonwoven fabric the higher is the relatively difference in pressure between suction device and the nonwoven surface. This can be counteracted by increasing the suction capacity, but this has the disadvantage that, on the one hand, the energy consumption is increased and, on the other, the lower portions of the mineral wool nonwoven fabric are compressed to such an extent that the mineral wool nonwoven fabric leaves the shroud already precompacted. Such a density gradient within the thickness of the insulant is undesirable since this reduces the insulance and other quality data such as e.g. pliancy and compressive stress of the product.
To obtain a bulk density distribution across the thickness of the product which is as even as possible the thickness of the raw nonwoven upstream of the curing oven needs to correspond to at least twice the product thickness.
It is known from experience that the thickness of the raw nonwoven upstream of the curing oven considerably effects the density distribution and thus the pliancy of compressed products.
Prior art attempts of reducing the density gradient across the thickness of the insulant involved directing an air flow firstly from the bottom upwards in the drying oven configured as a circulating air oven so as to loosen up the higher density lower layers.
Proposed in German patent 39 21 399 is an apparatus in which the collection conveyor is configured so that the carrying surface area of the collection conveyor increases in each case in the conveying direction. This is achieved by inclining the collection conveyor from the horizontal so that the suction surface area is increased and a lower vacuum is needed in this zone.
EP 0 406 107 too, describes one such method for depositing fibers generated by a plurality of fiberizing means. In this arrangement each fiberizing means has its own interceptor zone and the intercepted fibers are discharged from the interceptor zone by conveyor belts. The web of the conveyor belts is convex and the surface areas of the interceptor zones become larger with increasing surface area weights on these conveyor belts. The disadvantage of such an apparatus is that the rotary walls surrounding the shroud in this system are not configured down to the conveyor belts, thus resulting in leakages which increase the blower capacity needed. This is why a fixed wall section adjoins the rotary walls downwards. These fixed sidewalls result in the product being more exposed to dirt which tends to collect in these areas to then periodically drop onto the conveyor belt located underneath. This has the further disadvantage that these random dirt droppings negatively effect the consistent quality of the resulting product.
In addition there is an optimum to the spacing between the fiberizing means and the collection conveyor. If the spacing is too small, strong horizontal air flows materialize on the collection conveyor which tend to roll up the deposited fibers into bundles. If the spacing is too large largish bundles of fibers (also termed hanks) tend to already form in the collection shroud which likewise render the product inhomogenous.
To minimize both effects a precise spacing as calculated or obtained from trial and error needs to be maintained between fiberizers and collection conveyor.
U.S. Pat. No. 4,463,048 describes a method and an apparatus for producing mineral wool nonwovens comprising several fiberizing units which deposit the fibers as a primary nonwoven on a collection conveyor which conveys horizontally in the region of the fibers being deposited. The conveyor belts of the collection conveyor are then guided over guide pulleys so that a secondary nonwoven materializes from two primary nonwovens. Since two primary nonwovens having half conveyor contact are generated the resistance coefficient in the throughflow of the primary nonwovens is roughly half that for a secondary nonwoven twice as thick, enabling the vacuum needing to be applied by the blower to be reduced roughly by 50%. However, the various fiberizing means need to be set very precisely to minimize the differences in density due to the transverse distributions. Setting the fiberizing means with high accuracy is also important to minimize the property differences, especially as regards their symmetrical distribution across the thickness of the secondary nonwoven.
Described in U.S. Pat. No. 4,917,750 is an apparatus and a method in which the mineral wool nonwoven fabric is sliced along a generally horizontal plane prior to entering a curing oven. The upper section is compressed and subsequently redeposited on the lower section so that the resulting mineral wool product has density distribution across the thickness, whereby the upper portion has a higher thickness than the lower portion located thereunder. The slice through the mineral wool nonwoven fabric is made in a substantially horizontal plane, parallel to the transport belt.
Known from U.S. Pat. No. 3,824,086 is producing a very thick mineral wool nonwoven fabric in making use of various fiberizing means. In this arrangement each of the individual fiberizing means deposits on an individual conveyor belt assigned to it. The individual mineral wool sections thus produced are subsequently deposited on each other.
The invention is based on the object of improving an apparatus and a method for producing a mineral wool nonwoven fabric so that a product having enhanced properties can be produced with low energy consumption.
The substantial feature of the mineral wool blanket or mineral wool mat is that two layers in each case feature identical properties as regards fiber quality and/or binder content. As already explained above, each single fiberizing means produces two layers in the secondary nonwoven which are in addition symmetrical as regards the plane of symmetry running parallel to the top and bottom side of the mineral wool product, thus permitting a finer graduation of the density distribution than hitherto possible in prior art. In this context it is understood that identical properties are properties which deviate only slightly for a single fiberizing means whilst the term xe2x80x9cfiber qualityxe2x80x9d identifies both fineness and length of fiber responsible for the mechanical properties of the corresponding insulants.
The gist of the invention is based on providing a double-width shroud requiring a lower blower capacity since a mineral wool nonwoven fabric having half the unit weight is deposited in each case of the conveyor means. Fiber fineness and unit weight dictate the flow resistance of the raw nonwoven. In addition a means is provided for slitting the blanket of insulant into two blankets. From the double-wide designed shroud producing a mineral wool blanket having twice the width of that of the desired product, two separate mineral wool blankets are obtained, each of which has the width as required for producing the desired product. To marry the two blanket sections a conveying device is used which is able to guide the first blanket section so that it is deposited on the second blanket section.
Since the conveying distances of the generated blanket sections differ in length, fluctuations in the transverse distribution and fiber distribution of the mineral wool blanket generated in the shroud, as well as fluctuations in density are equalled out. To achieve this advantage even minor differences in the length of the conveying distances are sufficient, preferably greater than or equal to the fiberizer spacing in the shroud.
One substantial advantage of the apparatus in accordance with the invention is that two layers per fiberizing means are generated in the secondary nonwoven, meaning that the properties of the product are generated more symmetrically whilst significantly simplifying the setting between several fiberizing means. When, for example, as in U.S. Pat. No. 4,463,048 or, however, also in DE 39 21 399 C2, secondary nonwovens are produced from two primary nonwoven deposits, the skins of the secondary nonwoven are generated by various fiberizing means. This means that two different fiberizing means need to be set so that the fiber qualities, i.e. fiber fineness and length are practically the same, whereas in the apparatus in accordance with the invention two separate layers are formed in each case in the secondary nonwoven by one and the same fiberizing means and the two layers are arranged in each case in the product so that they are arranged symmetrical to the centerplane running parallel to the top and bottom side of the product.
In addition, the transverse distributions of individual fiberizing means are equalled out. Experience has shown that products of poor quality usually exhibit inadmissibly high transverse distributions of the discharged fiber quantity. Such transverse distributions are equalled out in the apparatus in accordance with the invention as explained in detail below.
Although in the double-wide shroud a mineral wool nonwoven fabric is produced having a lower unit weight and thus also exhibiting a low density gradient across the thickness of the mineral wool nonwoven fabric, it can never be excluded with absolute certainty that density gradients occur in conventional production of mineral wool nonwoven fabric in making use of a shroud above a conveyor means. As explained above, the density is highest at the underside of the mineral wool blanket. Despite this increase in density having the disadvantage that the heat insulating properties relative to the mass are diminished in this portion, these portions of higher density have the advantage of improved stiffness.
By inverting the first blanket section as preferred the first blanket section and second blanket section are married so that the corresponding higher density portions are located at the top and bottom of the blanket thus enabling products to be achieved having an enhanced dimensional stability for a low average density of the mineral wool blanket and the associated good insulating properties.
The apparatus in accordance with the invention has in addition the advantage that existing shrouds can be easily retrofitted. Since a usual conveyor means is arranged at the bottom of the shroud, whose distance from the fiberizing means is predefined, the fabricated raw nonwoven leaves the shroud level with the adjoining production line. When, instead, prior art drums or conveyor means at the bottom of the shroud are retrofitted the raw nonwoven leaves the system at a significantly lower level and thus first needs to be returned to the production line. However, the simple configuration of the shroud has, in addition, the advantage that any dirt collecting in the region of the conveyor means does not result in the product being contaminated.
With the apparatus as well as in making use of the method in accordance with the invention a blanket or mat of mineral wool can be produced from homogenous mineral wool fibers which has a density distribution across the thickness of the mineral wool blanket, it being understood in this context that the thickness is the dimension extending perpedicular to the width and also to the length of the mineral wool blanket fabricated and accordingly also perpedicular to the top and bottom of the mineral wool mats fabricated. In this arrangement the density distribution is configured so that in a smooth profile of the density across the thickness of the mineral wool product a higher density in the lower portion of the mineral wool product is initially continually diminished before then translating into a substantially continual portion in the middle and then continually increasing again in the upper edge portion to achieve a maximum value at or near to the top edge corresponding to the maximum value at or near to the bottom edge. This characteristic density distribution of the mineral wool product makes is easier to process due to the enhanced dimensional stability near to the top and bottom side whilst ensuring good heat insulating properties due to the even density in the middle portion. The desired uniform density across the thickness of the product is positively influenced by the staggered pile of the two blankets. In addition any asymmetrical fiber distribution problems can be offset by piling the two blankets, resulting in better mechanical properties of the product for the same average bulk density.
The substantial feature of the mineral wool blanket or mineral wool mat is that two layers in each case feature identical properties as regards fiber quality and/or binder content. As already explained above, each single fiberizing means produces two layers in the secondary nonwoven which are in addition symmetrical as regards the plane of symmetry running parallel to the top and bottom side of the mineral wool product, thus permitting a finer graduation of the density distribution than hitherto possible in prior art. In this context it is understood that identical properties are properties which deviate only slightly for a single fiberizing means whilst the term xe2x80x9cfiber qualityxe2x80x9d identifies both fineness and length of fiber repsponsible for the mechanical properties of the corresponding insulants.
The dimensional stability of the products depends not only on the bulk density but also on the binder content. Since a high binder content in the product has a negative effect on the fire resistance properties it is very important to restrict the portion of high binder content to the necessary edge zones. This effect too can be set particularly well by the arrangement as described. Since only one fiberizer forms each surface of the product, the portion of high binder content as well as portions differing in fiber quality, e.g. longer or finer fibers, can be set substantially more accuracy than in all other prior art devices a methods of production.
Preferred embodiments of the invention are characterized by the remaining claims.
In accordance with one preferred embodiment the means for slitting the insulant blanket generates a water cutting jet which can be directed at the insulant blanket. Making use of a water cutting jet has proven to be particularly more favorable than other cutting means, for example in the form of circular saws. In the region where it is parted the insulant blanket is still to harden and aggregate with the binder still tacky so that making use of such a water knife has the great advantage that working items used in parting cannot become stuck or detrimented. In addition densification of the nonwoven at the cut edge is avoided.
An alternative means for slitting the insulant blanket uses a laser beam.
When a high output is required several fiberizing means may be arranged staggered in both the conveying direction of the conveyor means and transversely to the conveying direction of the conveyor means.
In accordance with another preferred embodiment the conveyor means comprises for inverting the first blanket section an inverter pulley about which the first blanket section can be guided. This represents the simplest solution technically for inverting the first blanket section in accordance with the invention prior to it being deposited on the second blanket section. Due to the good coherency of the fabricated mineral wool blanket there is no risk of the produced mineral wool nonwoven fabric tearing in passing it about an inverter pulley, this being the reason why there is no need for further complicated technically means.
In accordance with yet another preferred embodiment the conveyor means of the primary nonwoven in the shroud is located substantial perpedicular to the conveying direction of the secondary nonwoven. This facilitates retrofitting existing production systems with minimum downtime by the preparatory work as regards the fiberizing means, shroud the most of the conveyor means required being implemented on an existing linear production line parallel to on-going operation.
Preferably the difference between the conveying distance of the first blanket section to the conveying distance of the second blanket section equals or exceeds the spacing of the fiberizing means. This simple geometry definition effectively assists compensating existing transverse distributions of the fiberizing means by a sufficiently high difference in the conveying distances being made available so that the maldistribution effects of the bulk densities of a single fiberizing means cannot accumulate undesirably.
With the method and apparatus in accordance with the invention mineral wool products can be produced which exhibit an average density of but 4 to 11 kg/m3, preferably 6 to 9 kg/m3. In addition a secondary nonwoven can be fabricated which already has mechanical properties prior to entering the curing oven so that it is not compressed in the thruflow of the curing oven. In the curing oven the secondary nonwoven runs between an upper belt and a lower belt as a result of which the thruflow of the drying air is zoned from top to bottom and bottom to top. It is usually so, due to the flow resistance of the nonwoven (product) in a bottom to top thruflow, that an air cushion is formed between the lower belt and the product whilst in the region of the upper belt hardening of the product already takes place to consolidate the product relative to its thickness. Subsequent thruflow of the curing oven from top to bottom then results in an air cushion being formed between the mineral wool product and the upper belt so that the product leaves the curing oven 20 mm to 40 mm less thick than the spacing between upper belt and lower belt. The raw and secondary nonwoven in accordance with the invention has such high mechanical properties that it is not compressed in the thruflow of the curing oven.